This seems to happen fairly regularly. I typically edit a set of four type styles at once, and open and close glyph edit windows frequently. I show kerning in the glyph edit window, but that also fails to work although the kern feature is enabled.
Restarting FontCreator resets the OpenType feature selections to none, and that fixes the issue.
Selecting a Script and Language in the Preview Toolbar triggers this bug. Setting the script back to Auto fixes the issue.
Balava Regular.fcp (480 KB)